Adjacent internal angles of a parallelogram add up to 180°. This means that 20x = 180.
So, we can divide 180 by 20 and we have x.
x = 180/20
x = 9
We can just multiply x by the other angle values now.
15x = 135
5x = 45
We know this is true because 135 and 45 add up to 180.